In general, aluminum flake pigments include leafing type and non-leafing type pigments. The leafing type aluminum flake pigment is obtained by employing a grinder such as a ball mill having spherical grinding media of steel such as steel balls and wet-grinding aluminum powder in an organic solvent such as mineral spirit or solvent naphtha with a grinding lubricant such as saturated fatty acid such as stearic acid. The non-leafing aluminum flake pigment is obtained by performing similar grinding with unsaturated fatty acid such as oleic acid as a grinding lubricant in place of the aforementioned saturated fatty acid or the like.
The leafing type aluminum flake pigment has weak affinity for a solvent or a binder contained in a paint due to small surface tension. Therefore, the leafing type aluminum flake pigment floats on the surface side of a coat and is substantially homogeneously oriented, thereby exhibiting excellent metallic effect and a base masking property.
On the other hand, the non-leafing type aluminum flake pigment has strong affinity for the solvent or the binder contained in the paint since the surface tension thereof is not so small. Therefore, the non-leafing type aluminum flake is distributed in a coat in a substantially homogeneously oriented state, thereby supplying the coat with metallic effect and exhibiting the so-called metallic design.
While the metallic effect of the coat is visually recognized in a combination of luminousness, brilliance, glitter etc., there is a strong tendency of desiring a coat having high luminance in general. In particular, an aluminum flake pigment having high luminousness has recently been highly required as a paint pigment for a talc can such as a beverage can. In general, the luminance of a coat and the average particle diameter of an aluminum flake pigment are so correlated that the luminance is increased as the average particle diameter is increased.
If the aluminum flake pigment has a large average particle diameter, however, the orientation of the aluminum flake pigment tends to be disturbed in formation of the coat, the aluminum flake pigment may project from the coat to result in blobbing on the surface of the coat, and the glitter of the coat may be too strong to result in unpreferable design.
Therefore, development of an aluminum flake pigment having a small average particle diameter as well as high luminance is desired and many efforts for development are made in various fields.
In relation to the leafing type aluminum flake pigment, various developments are made as those capable of forming a coat exhibiting high reflectance represented by electroplated chrome finishing, for example, and a paint having strong brightness employing a leafing type aluminum flake pigment of 0.1 to 1 μm in thickness and 1 to 60 μm in average particle diameter, for example, is disclosed (refer to Japanese Patent Laying-Open No. 2001-240808, for example).
A technique related to a paint employing a leafing type aluminum flake pigment having a small particle diameter is also disclosed (refer to Japanese Patent Laying-Open No. 2001-81359, for example). It is described that this paint has metallic effect with brightness substantially identical to that of an evaporated film and is preferable for a reflector of an automobile identification lamp such as a headlamp, a signal lamp or a tail lamp.
In relation to the non-leafing type aluminum flake pigment, it is disclosed that a metallic pigment composition containing (A) 100 parts by solid weight of coat forming resin and (B) 0.1 to 30 parts by weight of an aluminum flake pigment having an average particle diameter (D50) in the range of 20±5 μm, an average particle thickness (t) in the range of 0.5 to 1 μm and a gradient (n) of at least 2.7 in a Rosin-Rammler diagram can simultaneously supply a coat with strong luminousness and excellent appearance (refer to Japanese Patent Laying-Open No. 8-170034, for example).
Further, an aluminum flake pigment having high luminance and rich circulation resistance with an average thickness of 0.2 to 0.7 μm, an average particle diameter of 4 to 20 μm, an aspect ratio of 15 to 50 and a homogeneity factor (n)≧2.4 is disclosed (refer to Japanese Patent Laying-Open No. 11-152423, for example).
Also in paints employing these conventional leafing type aluminum flake pigments and non-leafing type aluminum flake pigments, however, the requirement for an aluminum flake pigment having a small average particle diameter as well as high luminance is not sufficiently satisfied.